Lead-free enamel composition, the corresponding enamels and glass-ceramic articles, a new lead-free mineral glass

ABSTRACT

Objects of the present invention are: 
 
a lead-free enamel composition comprising finely divided glass particles, finely divided pigment particles and an organic binder, characterised in that the glass of said particles is a lead-free mineral glass, essentially having the composition below, which is expressed in percentages by weight of oxides:  
                                                   SiO 2     45-60   CaO    0-12         B 2 O 3      0-10   BaO   13-27         Al 2 O 3      6-17   ZnO    3-17         Na 2 O   0-7   MgO   0-9         K 2 O   0-7   TiO 2     0-2         Li 2 O   0-7   ZrO 2     0-7         with Na 2 O +   &gt;4         K 2 O + Li 2 O                                     
the enamel able to be obtained by firing of said composition; glass-ceramic articles, which are decorated with said enamel; and new lead-free mineral glasses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of French Application No. 01 09080,filed Jul. 9, 2001.

The invention relates to an original lead-free enamel composition, tothe corresponding enamel and glass ceramic materials, as well as to anew lead-free mineral glass.

More specifically, the invention relates to:

said lead-free enamel composition containing particles of a lead-freemineral glass;

the enamel able to be obtained by firing of said composition;

glass-ceramic articles decorated with said enamel, able to be obtainedfrom said enamel composition;

a new lead-free mineral glass particularly suitable as constitutiveelement of said enamel composition;

the use of some lead-free mineral glass for manufacturing efficientenamels.

BACKGROUND OF THE INVENTION

In U.S. Pat. No. 4,883,705, some lead-free mineral glass compositionsare also described within the context of the manufacturing of ceramicsubstrates used for electronic devices. Said compositions arecharacterized by a relatively low SiO₂ content (38 to 48% by weight), arelatively high ZnO content (10 to 20% by weight), a relatively high CaOcontent (1 to 8% by weight) and a relatively low Li₂O+Na₂O+K₂O content(0 to 5% by weight).

Most of the commercial glass-ceramic cooktops are decorated with the aidof variously coloured enamels. Such a decoration on the one handconforms to aesthetic criteria, which imply a large variety of tints andmotifs, and on the other hand, aim to attain the objective of safety, inoutlining with these decorations the location of the areas which heatand which, during the functioning of the cook top, are susceptible toattaining high temperatures.

The preparation of the enamels is normally done in several steps. Firstof all, a glass is melted according to classical glass industrytechniques. This glass is then ground to a particle size which iscompatible with the subsequent technique of application of the enamelonto the support to be enamelled (an application technique whichgenerally consists in screen printing or decal(comania)). Pigmentsand/or opaque-making agents are added to the glass powder which areintended to confer to the final enamel its tint and its opacity. Theresulting powdered mixture (glass powder+pigments and/or opaque-makingagents) is finally incorporated into an organic binder, in proportionswhich are adjusted to the viscosity desired for the resulting paste. Theenamel composition thus obtained is ready for use.

The enamel composition is applied onto the substrate concerned, bydirect screen printing, by transfer, by decal(comania) or by any otherprocess, and then dried and fired by an appropriate heat cycle, whichtypically comprises a plateau at a temperature of greater than 900° C.

Beyond the two functions set forth above, (with reference to theaesthetics and to the safety), the enamel decorations must meet thephysico-chemical requirements which arise from the conditions of use ofthe glass-ceramic cooktops, cooktops on which said decorations generallyare incorporated. Thus, the decorations must be able to resist attack byacids and bases; they must be easy to clean, even after calcination offood remains; they must have a good resistance to abrasion, to frictionwith metallic pieces; they must not be sensitive to wrenching. Theincorporation of said decorations must not either affect, too much, themechanical strength of the tops on which they are incorporated. To thepresent day, certain tops on the market are susceptible to being broken,when a pressure of the order of 70 to 80 MPa is exerted on the enamelledface. Under certain conditions, especially of extreme transport and ofstorage, a resistance to a greater pressure (120-140 MPa) is requiredfrom the tops.

Traditionally, the introduction of lead oxide into the glass composition(entering in the composition of the enamel) strongly lowers itsviscosity and thus guarantees a good coating, i.e. a good moistening anda good covering of the glassceramic by the enamel. However, in view ofthe present tendency to remove toxic elements from products used incooking, we consider that the enamels used for the decoration ofglass-ceramic cooktops must be free from such elements, and inparticular free from heavy metals, such as lead.

According to prior art, within the context of a search for veryefficient products, numerous lead-free glass compositions and enamelcompositions which incorporate said glasses have been described.

In FR-A-2 686 333, glass compositions are described which are lead-freeand cadmium-free and which are intended for vitrification, enamellingand decoration. Said compositions contain no lead, no cadmium, no zinc,no tin, no fluorine. They can in particular contain barium, between 0and 12% by weight of barium oxide. Finally, they are characterised by arelatively high CaO content (3 to 18% by weight).

The Applicant has itself, in FR-A-2 732 960, described lead-free enamelcompositions which comprise finely divided glass particles, finelydivided pigment particles and an organic binder, which are characterisedin that the glass has a thermal dilation coefficient less than or equalto 55 10⁻⁷/° K and has the following composition, in percentages byweight, on the basis of the oxides: SiO₂ 45-60 B₂O₃  0-10 Al₂O₃  6-17Li₂O 0-2 Na₂O 0-3 K₂O 0-3 Li₂O + Na₂O + K₂O <4 CaO  0-12 MgO 0-9 ZnO 0-17 BaO  0-27 SrO  0-16 CaO + MgO + ZnO + BaO + SrO 22-42 TiO₂ 0-2ZrO₂ 0-7said glass having a softening temperature T_(L) greater than 680° C.

The Applicant has pursued its research work and is now in a position tobe able to provide novel lead-free enamel compositions containingparticles of lead-free mineral glass and the corresponding novelenamels.

DETAILED DESCRIPTION OF THE INVENTION

The enamels of the invention, which are classically made from saidlead-free glasses, are particularly efficient in terms of resistance tofriction with metallic pieces, and of resistance to wrenching.Furthermore, said enamels affect only slightly the mechanical strengthof the glass-ceramic tops on which they can be disposed.

Said enamels of the invention are in fact analysed as improvements tothose described in FR-A-2 732 960.

According to its first object, the present invention relates tolead-free enamel compositions which contain, classically, finely dividedglass particles (generally of an average particle size of less than 5μm), finely divided pigment particles, and an organic binder.Characteristically, said enamel compositions comprise particles ofspecific mineral glasses, which are lead-free (as well as free from anyother toxic metal), and which are more particularly suitable for themaking of enamels. Said mineral glasses essentially have the followingcomposition by weight, (i.e. expressed in percentages by weight on thebasis of the oxides): SiO₂ 45-60 B₂O₃  0-10 Al₂O₃  6-17 Na₂O 0-7 K₂O 0-7Li₂O 0-7 with Na₂O + K₂O + Li₂O >4 CaO  0-12 BaO 13-27 ZnO  3-17 MgO 0-9TiO₂ 0-2 ZrO₂ 0-7

Silica is a classical component of the mineral glasses. It plays apredominant role on the properties such as the hardness of the glass andits chemical durability. Its content, within the used glasses, isgreater than or equal to 45% so as to obtain the result expected inthese terms of hardness and chemical durability. Its content does notexceed 60% in order to enable a correct fusion and a correct coating ofthe enamels incorporating said glasses.

B₂O₃ is generally incorporated within the glass in order to enable theviscosity of the enamel (prepared from said glass) to be lowered andtherefore to enable a good coating on the substrate to be decorated tobe guaranteed, while at the same time not causing an exaggeratedincrease in the coefficient of dilation. The quality of the coatingdirectly influences the ease of cleaning of the enamel, which is betterfor smooth surfaces. B₂O₃ further improves the resistance to abrasion ofthe enamel. B₂O₃ must not be incorporated in excess (above 10%), sincethen an undesirable iridescence can appear around the enamel (enamelincorporating the said glass) decoration elements after firing of it.B₂O₃ is more often incorporated at the rate of 5 to 6% by weight. Itdevelops a noticeable action from and above 0.5% by weight.

Al₂O₃ is incorporated at a concentration greater than or equal to 6%, soas to increase the hardness of the enamel. The alumina increases themicro-hardness of the enamel and therefore, especially, its resistanceto scratching. Above 17% of Al₂O₃, the viscosity of the enamel becomestoo high and jeopardises a correct coating of it.

Used individually or in combination, Li₂O, Na₂O and K₂O facilitate themelting of the glass and lower the viscosity of the enamel incorporatingsaid glass (which thus favours its coating). These three oxides can beincluded, each one, up to 7% by weight and are incorporated, in anycase, together, at more than 4% by weight. This teaching of theinvention contradicts that of the application FR-A-2 732 960. TheApplicant has presently established the positive consequence (notably interms of resistance to wrenching and to friction with metallic pieces)of this higher content of oxide(s) of alkali metal(s). Within thecontext of an advantageous variant of the invention, it is recommendedto incorporate said alkali metals at more than 4.5% by weight and moreadvantageously at more than 5% by weight:

-   -   advantageously Na₂O+K₂O+Li₂O>4.5; more advantageously >5.

CaO can be present to improve the chemical durability of the glass andthe coating of the enamel incorporating said glass. It develops anoticeable action from and above 0.5% by weight. Its concentration inthe composition of the glasses of the invention must not exceed 12%, inorder to maintain a good resistance to abrasion. This oxide isadvantageously incorporated at a low content, between 0 (0.5%) and 2%and more advantageously between 0 and <1%.

BaO is always present, in a relatively high content, from 13 to 27%. BaOthus guarantees a good chemical durability of the enamel whichincorporates the glass. BaO is thus incorporated in order to improve theresistance to abrasion of the enamel. Used in a higher amount, itjeopardises the coating of the enamel.

ZnO is also always present, at a content between 3 and 17%,advantageously between 5 and 17% and more advantageously between 5 and<10%. Excessive amounts of ZnO can jeopardise the softening temperatureof the glass, then of the enamel. ZnO confers resistance to abrasion tothe enamel.

MgO, similarly, can confer a better resistance to abrasion to theenamel. MgO develops a noticeable action from and above 0.5%. Usedbeyond 9%, MgO can jeopardise the chemical durability of the enamel.

TiO₂ is suited to harden the final enamel. It plays a very positive rolein the chemical durability of said enamel.

ZrO₂ is similarly suited to harden the final enamel. It also plays avery positive role in the chemical durability of said enamel. It doesnot increase the dilation of the glass. Its incorporation is limited toa maximum value of 7%, a value above which it becomes difficult to meltthe glass without non-melted solid inclusions.

The essential constituents of the glasses of the enamel compositions ofthe invention—SiO₂, Al₂O₃, Na₂O and/or K₂O and/or Li₂O, BaO and ZnO—areadvantageously incorporated in combination with at least one optionalconstituent—B₂O₃, CaO, MgO, TiO₂, ZrO₂—. The minimum amount ofincorporation of said optional constituents is obviously that at whichit develops an effect. This minimum amount is generally situated at0.5%.

In addition to the essential and optional constituents set forth above,the glasses of the enamel compositions of the invention can containother constituents. Obviously, they contain such constituents only in alimited amount (less than 5%, generally less than 2%), only in an amountwhich does not jeopardise the characteristics of the glasses andtherefore the ones of the enamels of the invention. It is so in no wayexcluded that the glasses of the enamel compositions of the inventioncontain strontium oxide (SrO).

Particularly preferably, the glasses of enamel compositions of theinvention essentially have the composition below, which is expressed inpercentages by weight of oxides: SiO₂ 45-55 B₂O₃ 4-6 Al₂O₃ 6-8 Na₂O 2-4K₂O 2-4 Li₂O 1-3 CaO 0-2 BaO 16-19 ZnO  6-10 MgO 3-5 TiO₂ 0-2 ZrO₂ 1-3Within the said preferred composition, advantageously:

-   -   Na₂O+K₂O+Li₂O>5; and/or    -   CaO 0−<1.

As regards the pigments (and/or opaque-making agents) and the binderswhich can be incorporated within the lead-free enamel compositions ofthe invention, in the making of said enamels, these are conventionalpigments (and/or opaque-making agents) and binders, which areconventionally incorporated.

Classically, white enamels are obtained by the addition of cerium oxideand/or titanium oxide, the average particle size of which is of theorder of and generally less than 1 μm, not only with reference to thecoloration power and opaque-making power of said oxides, but also inorder to guarantee a good resistance to the abrasion of the final enamel(in order to minimise, even to prevent, any receding of pigment grainsout of said final enamel). The use of titanium oxide is particularlyrecommended.

Other oxides, such as those of iron, chromium, cobalt, zinc, manganese,alone, in a mixture, or added to the cerium oxide and/or titanium oxide,are used in order to obtain coloured enamels, notably brown enamels.

The amount of addition by weight of the pigments to the basic glass istypically of the order of 10 to 40% (by weight) according to the colourand opacity desired.

Any type of organic binder can be incorporated in the composition of theinvention, and in particular binders which are based on pine oil or onacrylic resins, at contents which typically range between 30 and 60% byweight. The proportion of incorporation of such a binder determines theviscosity of the paste and enables the thickness of the enamel afterfiring to be adjusted.

The enamel compositions of the invention, which are heat treated,classically, generate the enamels of the invention, which constitute thesecond object of said invention.

Said enamels are more particularly suitable for the decoration ofglass-ceramic articles. Said articles, decorated with the enamels of theinvention obtained from an enamel composition of the invention,constitute the third object of said invention.

Notably, these glass-ceramic articles are glass-ceramic cooktops. It hasalready been understood that the invention has in particular beendeveloped in this context.

Said enamels, which are disposed on a glass-ceramic top, maintain anacceptable resistance of the enamelled face, have good chemicaldurability characteristics, good cleanability characteristics, and goodwrenching resistance characteristics, and good characteristics ofresistance to abrasion.

Among the glasses having the composition specified above—glasses whichare constitutive elements of the enamel compositions of theinvention—some are novel and constitute a fourth object of saidinvention. The said fourth object consists of the mineral glassesessentially have the following composition by weight, (i.e. expressed inpercentages by weight on the basis of the oxides): SiO₂ 45-60 B₂O₃  0-10Al₂O₃  6-17 Na₂O 0-7 K₂O 0-7 Li₂O 0-7 with Na₂O + K₂O + Li₂O >5 CaO 0-12 BaO 13-27 ZnO  3-17 MgO 0-9 TiO₂ 0-2 ZrO₂ 0-7

Advantageously said mineral glasses have the composition indicated abovewith the ZnO content between 5 and 17% by weight and more advantageouslybetween 5 and <10% by weight.

Advantageously said lead-free mineral glasses have the compositionindicated above with CaO content between 0 and 2% by weight and moreadvantageously between 0 and <1% by weight.

Particularly preferably, the glasses of the invention essentially havethe composition below, which is expressed in percentages by weight ofoxides: SiO₂ 45-55 B₂O₃ 4-6 Al₂O₃ 6-8 Na₂O 2-4 K₂O 2-4 Li₂O 1-3 withNa₂O + K₂O + Li₂O >5 CaO 0-2 BaO 16-19 ZnO  6-10 MgO 3-5 TiO₂ 0-2 ZrO₂1-3

The original glasses provided are more particularly interesting whenused as a raw material in the manufacturing of enamels (see above).

Lastly, the invention concerns, according to its fifth object, the useof a lead-free mineral glass essentially having the composition below,which is expressed in percentages by weight of oxides: SiO₂ 45-60 B₂O₃ 0-10 Al₂O₃  6-17 Na₂O 0-7 K₂O 0-7 Li₂O 0-7 with Na₂O + K₂O + Li₂O >4.5CaO 0-2 BaO 13-27 ZnO  3-17 MgO 0-9 TiO₂ 0-2 ZrO₂ 0-7for manufacturing a lead-free enamel.

EXAMPLES

The invention is illustrated by the Examples below (Examples 4 to 7).The usefulness thereof emerges from the consideration of said Examples 4to 7 and of that of Comparative Examples 1 to 3. Examples 1 and 2illustrate the teaching of FR-A-2 732 960, Example 3 illustrates theeffect of the non-incorporation of ZnO in the composition of the glass.Examples 5 to 7 illustrate glasses and enamel compositions of theinvention. Example 4 illustrates another enamel composition of theinvention.

Table 1 below presents a group of lead-free glass compositions whichenable the preparation of enamels for decoration of glass-ceramic tops.These glass compositions are expressed, in percentages by weight, on thebasis of the oxides.

It is to be noted that it is possible for the glasses to be melted fromusual starting materials, in the form of oxides or other compounds,which by decomposition get converted into oxides, in the proportionsdesired. For example, lithium carbonate and sodium nitrate can be usedas sources of lithium and sodium, respectively.

The glasses are melted at 1,550° C., in an amount which is sufficient inorder that after grinding, sufficient material be available for theremainder of the process. At the end of the melting cycle, i.e.typically after 5 hours at the above-mentioned temperature, a part ofthe glass is poured into water, then dried in an oven, while theremaining part is poured in the form of a top, in order to enable thecharacterisation of the glass.

The glass poured into the water is in the form of unshaped fragmentswhich are dried, ground and sieved in several steps, until asufficiently fine average particle size (less than 5 μm) for the furtherenamel application process be obtained. The grinding is carried out, forexample, with the aid of aluminium bead grinders, either dry or in analcoholic medium. The powder collected is then dried and undergoes acheck of the particle size.

At this stage, the pigments are added, mixed with the glass powderobtained previously. The whole obtained is optionally ground again. Thenature and the amount of these pigments depend upon the colour soughtfor the final enamel.

The pigments used within the context of these Examples are marketed bythe company DMC² under the denominations X 928, B 768, EV1082. Thefollowing was in fact added:

-   -   12% by weight of X 928    -   10% by weight of B 768    -   8% by weight of EV 1082,        to 70% by weight of sintered glass for Examples 1, 2 and 5;    -   6% by weight of X 928    -   5% by weight of B 768    -   4% by weight of EV 1082,        to 85% by weight of sintered glass for Examples 3, 4, 6 and 7.

The last step of preparation of the enamel consists in incorporating anorganic binder into the powdered mixture of the glass and the pigments.

Within the context of the Examples, a type of organic binder isincorporated which is pine oil-based. The enamel compositionsconstituted contain in the order of 50% by weight of said binder.

The application of said enamel compositions onto a glass-ceramicsubstrate was made by direct screen printing. The tops used are thosemarketed by the Applicant, EUROKERA, under the trademark Kérablack,described in U.S. Pat. No. 5,070,045.

The firing of the enamel composition was then carried out according to aheat cycle comprising a plateau at a temperature of 925° C. for 20minutes. Upon completion of this cycle, the enamel layer typically has athickness of the order of 3 to 5 μm.

The enamelled tops were tested:

in terms of mechanical strength: their modulus of rupture (MOR:expressed in MPa) by means of a three-point setting apparatus, thedecorated surface being in extension;

they were also subjected to a normalised resistance to rupture test. Thepercentage of broken samples was recorded for a given impact energy(expressed in Joules), the impact due to a falling pan;

in terms of resistance to wrenching: an adhesive tape (Scotch® type) isstuck onto the enamel of the top an then wrenched off with a givenforce. It is then noted whether the enamel is wrenched;

in terms of resistance to friction with metallic pieces: the decoratedsurface is rubbed with various metallic pieces (pans or coins). Theintensity of the marks and the ease of removing them was then evaluatedby using commercial products which are sold specially for cleaning thistype of top (glass-ceramic tops decorated with enamelled motifs). Theoverall result is marked up to 20. The better the mark, the greater theresistance to friction by metals is.

The results of said tests are given in the second part of Table 1 below.

The tops of the invention are more than suitable with reference to theother properties, developed in FR-A-2 732 960. TABLE 1 CompositionFR-A-2 732 960 INVENTION (% by weight) 1 2 3 4 5 6 7 SiO₂ 51.4 51.3 5550 49.5 49.1 48.6 B₂O₃ 5.5 5.5 6.9 5.5 5.4 5.4 5.4 Al₂O₃ 7.2 7.2 7.1 7.37.2 7.1 7.1 Na₂O 2 2.6 2.6 2.7 4.4 2.6 2.6 K₂O — — 3.3 — — 2.7 3.3 Li₂O1.3 1.2 1.3 2.1 1.3 1.3 1.3 CaO 5 0.6 0.6 0.6 0.6 0.6 0.6 BaO 13.7 1817.8 18.2 18 17.8 17.8 ZnO 7.1 8 0 8.1 8 8 8 MgO 3.1 3.8 3.8 3.8 3.8 3.83.8 ZrO₂ 1.7 1.7 1.7 1.8 1.8 1.7 1.7 % pigment used 30% 30% 15% 15% 30%15% 15% MOR (MPa) 122 149 130 145 122 154 140 % breakage after 0% at 3 J0% at 3 J 100% at 0% at 0% at 0% at 3 J 20% at impact at 2.3 J 3 J 3 J 3J (Joules) Wrenchings Some Some Some None None None None Mark (/20) ofthe 10 13 17.5 14 15 16.5 17.5 tests of friction with metallic pieces

The enamels of the invention are indeed more efficient than the enamelsof the prior art.

The enamels obtained from the glass compositions of the invention aremore efficient than the enamels obtained from the glass compositionsaccording to FR-A-2 732 960. They exhibit a notable gain in terms ofresistance to wrenching and resistance to friction with metallic pieces,without generating a degradation of the mechanical strength of the topson which they are incorporated (to enamel them).

Example 3 illustrates the necessity for the glass composition precursorto contain zinc. Sid zinc enables the enamelled top to present anincreased mechanical strength (evaluated notably by the test to measurethe percentage of breakage after impact) and simultaneously, enables theenamel to present a good resistance to wrenching and a good resistanceto friction with metallic pieces.

1-6. (canceled)
 7. Glass-ceramic article, comprising a decoration inenamel, wherein said enamel is an enamel according to claim
 16. 8. Theglass-ceramic article according to claim 7, characterised in that it isa cooktop.
 9. A lead-free mineral glass, essentially having thecomposition below, which is expressed in percentages by weight ofoxides: SiO₂ 45-60 B₂O₃  0-10 Al₂O₃  6-17 Na₂O 0-7 K₂O 0-7 Li₂O 0-7 withNa₂O + K₂O + Li₂O >4 CaO  0-12 BaO 13-27 ZnO  3-17 MgO 0-9 TiO₂ 0-2 ZrO₂ 0-7.


10. The mineral glass according to claim 9 essentially having thecomposition indicated with: ZnO 5-17.
 11. The mineral glass according toclaim 9 essentially having the composition indicated with: CaO 0-2. 12.The mineral glass according to claim 9, essentially having thecomposition below, which is expressed in percentages by weight ofoxides: SiO₂ 45-55 B₂O₃ 4-6 Al₂O₃ 6-8 Na₂O 2-4 K₂O 2-4 Li₂O 1-3 withNa₂O + K₂O + Li₂O >5 CaO 0-2 BaO 16-19 ZnO  6-10 MgO 3-5 TiO₂ 0-2 ZrO₂ 1-3.


13. (canceled)
 14. The mineral glass according to claim 9, wherein theweight percentage sum of Na₂O+K₂O+Li₂O is over
 5. 15. The mineral glassaccording to claim 11, wherein the weight percentage sum ofNa₂O+K₂O+Li₂O is over
 5. 16. An enamel consisting essentially of thelead-free mineral glass according to claim
 9. 17. The enamel accordingto claim 16, consisting essentially of the lead-free mineral glassaccording to claim
 12. 18. The glass-ceramic article bearing an enameldecoration according to claim 7, wherein said enamel is one according toclaim
 17. 19. The glass-ceramic article according to claim 18, whereinthe article is a cooktop.